1. Field of the Invention
The present invention relates generally to medical devices and, more particularly, to sensors used for sensing physiological parameters of a patient.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to certain aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the field of medicine, doctors often desire to monitor certain physiological characteristics of their patients. Accordingly, a wide variety of devices have been developed for monitoring many such characteristics of a patient. Such devices provide doctors and other healthcare personnel with the information they need to provide the best possible healthcare for their patients. As a result, such monitoring devices have become an indispensable part of modern medicine.
One technique for monitoring certain physiological characteristics of a patient is commonly referred to as pulse oximetry, and the devices built based upon pulse oximetry techniques are commonly referred to as pulse oximeters. Pulse oximetry measures various blood flow characteristics, such as the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and/or the rate of blood pulsations corresponding to each heartbeat of a patient. In fact, the “pulse” in pulse oximetry refers to the time varying amount of arterial blood in the tissue during each cardiac cycle.
Pulse oximeters typically utilize a non-invasive sensor that emits light into a patient's tissue and that photoelectrically detects the absorption and/or scattering of the transmitted light in such tissue. One or more of the above physiological characteristics may then be calculated based upon the amount of light absorbed or scattered. More specifically, the light passed through the tissue is typically selected to be of one or more wavelengths that may be absorbed or scattered by the blood in an amount related to the amount of a particular constituent present in the blood. The amount of light absorbed and/or scattered may then be used to estimate the amount of the blood constituent in the tissue using various algorithms.
The pulse oximetry measurement depends in part on the assumption that the contribution of light that has not passed through a patient's tissue is negligible. However, outside light may leak into a sensor, causing detection of light that is not related to the amount of blood constituent present in the blood. Additionally, shunted light or light from a sensor's emitter, may be reflected around the exterior of the tissue and may be sensed by the detector without traveling first through the tissue. These light sources may cause measurement variations that result in erroneous blood constituent readings.
Some outside light infiltration into the sensor may be avoided by fitting the sensor snugly against the patient's tissue. However, such a conforming fit may be difficult to achieve over a broad range of patient physiologies without adjustment or excessive attention on the part of medical personnel. Additionally, an overly tight fit may cause local exsanguination of the tissue around the sensor. Exsanguinated tissue, which is devoid of blood, may shunt the sensor light through the tissue, which may also result in increased measurement errors.
External light and shunted light may also be prevented from reaching the sensor by certain coatings applied to the pulse oximetry device. For example, some sensors incorporate reflective coating on the tissue contacting surface to reflect shunted light away from the detector. However, these reflective materials are metal-based, and thus conductive, which may result in capacitive coupling between the emitter and detector. In particular, conductive reflective materials may provide electrical paths between the pulse oximeter's light emitter and the detector. These electrical paths may cause corruption of the detector's measurement signal, resulting in an incorrect reading of more or less absorption of light than is actually transmitted through the patient's tissue. Therefore, noise added to the signal by crosstalk can lead to erroneous physiological measurements.